1. Field of the Invention
The present invention relates to optical switches and, in particular, to a polarization splitting switch.
2. Description of the Related Art
Optical switches have applications in integrated optics, fiberoptic communications, and detection systems. Optical switches which utilize light as the switching mechanism rather than using a mechanical mechanism, an electrical mechanism, a thermal mechanism, or the like, are known as all-optical switches. An all-optical switch switches an optical signal from one output port to another. This is accomplished by applying an input pump signal from a pump light source to cause the optical signal to be selectively switched. The switch is responsive to the pump signal to selectively switch the light of the optical signal to one or the other of the output ports.
The basic configuration for a typical all-optical switch is a Mach-Zehnder interferometer which includes a first fiberoptic input arm for receiving an input optical signal and a second fiberoptic input arm for receiving a switching pump signal. The input arms are fused together to form a first coupler which subsequently branches out into two intermediate arms. The first coupler splits the input light signal into equal portions which then enter the two intermediate arms. The two intermediate arms are once again fused to form a second coupler which branches into two output ports. After traveling through the two intermediate arms, the two signals are recombined by the second coupler. If the two signals are in phase at the second coupler, then all the light is coupled into a first one of the two output ports. If the two signals are completely out of phase, then the light is coupled into the other of the two output ports.
A number of difficulties have been encountered in the development and implementation of such all-optical switches. For example, one difficulty with the switch described above is that in the absence of a pump input, the phase bias due to the differing lengths of the two arms must be set to a very precise value (e.g., 0). This requires careful control of the relative length of the two arms to a fraction of a wavelength. For typical arm lengths of many centimeters or more, this has been found to be quite difficult. This problem is typically reduced by applying an external, steady-state phase shift using a phase modulator placed in one of the Mach-Zehnder fiber arms, thus setting the phase bias precisely to its desired value. This technique is well known in the art.
A second problem which has been encountered is that the bias introduced by a phase modulator is highly sensitive to external temperature variations. If the fiber lengths of the arms are not equal, even by only a few hundred wavelengths, and the average temperature of the device is changed, both the indices and the lengths of the two fiber arms change by different amounts which causes the phase difference due to the difference in the lengths of the propagation paths to also change. The differences are caused by the expansions and contractions due to temperature variations which are proportional to the lengths of the fiber so that a longer fiber will expand to a greater length than a shorter fiber, thus causing a phase imbalance. This phase imbalance modifies the signal power splitting ratio at the output ports of the Mach-Zehnder interferometer. Typically, such a change would be exhibited on a time scale comparable to the time it takes for the ambient temperature of the environment of the interferometer to change by a few degrees Fahrenheit.
The interferometer is even more sensitive to temperature gradients. For example, if the temperature of the two arms changes by differing amounts due to the temperature gradients between the arms, the signal power splitting ratio at the output ports again changes, but more rapidly.
Both of the above-described temperature-dependent effects, which are present whether the Mach-Zehnder interferometer is pumped or unpumped, are undesirable. In practice, these effects are reduced by making the fiber arms physically as close to each other as possible and by making the fiber arm lengths as equal as possible. However, in general, these measures are not sufficient to keep the Mach-Zehnder interferometer output (in the absence of a pump signal) stable to the degree necessary for many applications.
Therefore, another method which has been employed to actively stabilize the output coupling ratio of the interferometer is to use a control loop wherein the signal at one of the output ports is detected and compared to a reference to generate an error signal proportional to the difference between the reference and the detected output signal. This error signal is then amplified and fed into the same phase modulator that sets the bias so as to apply just enough phase to dynamically zero the error signal.
Although active stabilization of the bias works well, it is cumbersome, it increases the device cost, it requires access to the optical signal, and it leads to technical difficulties when the signal is dynamically switched. Most importantly, active stabilization requires electronic circuitry to run the switch, which is typically not acceptable for sensor array or other applications for which a minimization of electrical connections is desirable.
Another undesirable effect in a Mach-Zehnder interferometer switch is caused by the effects of the input pump power. More specifically, since the pump signal is applied only to one arm, heat is generated within that arm, and heat is not generated in the arm that does not carry the pump power. This temperature differentiation results in a pump-induced thermal phase shift that may cause an imbalance in the coupling ratio for the Mach-Zehnder interferometer when the pump is on. Because this effect is thermal, it is typically slow so that a few microseconds or more are required for this imbalance to vanish after the pump has been turned off. In some applications, this effect can be a significant problem.
In a Mach-Zehnder switch, thermal stability dictates that the two fiber arms have nearly identical lengths in order to minimize the effect of overall temperature changes. If the switch is to operate over a large temperature range (e.g., on the order of tens of degrees Fahrenheit), the splitting ratio of the two couplers forming the Mach-Zehnder interferometer switch should not vary with temperature. Furthermore, as discussed above, temperature gradients should be minimized.